1. Field of the Invention
The present invention relates to a method of determining a characteristic of a substrate.
2. Background Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor the lithographic process, it is necessary to measure parameters of the patterned substrate, for example the overlay error between successive layers formed in or on it. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. One form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Two main types of scatterometer are known. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. Angularly resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle.
The manufacture of IC chip involves the fabrication of many layers. In order to generate a more detailed pattern a plurality of lithography and etch processing steps may be used in the manufacture of each layer: this is known as double patterning. There are a number of different methods of achieving double patterning. The first of these is known as lithographic-etch-lithography-etch (LELE) and in this a first pattern is exposed and etched. A second pattern, with features located in the spaces between the features of the first pattern, is then exposed and etched. Thus, a pattern of smaller dimensions can be generated. Another similar double patterning technique is known as lithography-freeze-lithography-etch (LFLE). A pattern is exposed in the resist, which is then frozen. A second pattern can then also be exposed in the resist and both patterns are then etched into the substrate. Another double patterning method is known as the spacer method. In the spacer method a sacrificial template is put down and spacers placed either side, and adjacent to, the sacrificial template. The template is then removed and the resulting pattern etched into the substrate.
When two lithography steps are used to create a single pattern there may be some errors, for example in the placement of the features during the second lithography step. Similarly, the features exposed during the first lithography step may not be identical to those exposed during the second lithography step. As there have been two lithography steps the features exposed during each lithography step may be different and need to be assessed separately. However, as the features exposed during the first and second lithography step are, necessarily, very similar and form a regular pattern it can be difficult to distinguish between the two sets of features using angular resolve scatterometry.
In the spacer technique a spacer is used to generate a regular pattern. However, if the spacer is too large or too small the pattern will be irregular. Similarly, although the pattern may be almost irregular it will be difficult to assess small irregularities in the pattern.
Previously SEM has been used to assess the features exposed in each of the exposure steps. However, SEM is not fast enough to keep up with the throughput of substrate in high volume manufacturing of IC chips and can lack accuracy.